Retroreflective sheeting has the ability to redirect incident light towards its originating source. This advantageous property has led to the wide-spread use of retroreflective sheeting on a variety of articles. Typical examples of retroreflective sheeting are microsphere-based sheeting and cube-corner structured retroreflective sheeting.
Retroreflective sheetings are often provided with a cover sheet intended to protect the retroreflective elements from water, dirt, etc. and to provide effective optical arrangement for retroreflection. A typical embedded-lens retroreflective comprises a monolayer of microspheres embedded in a cover layer. Behind the rear surfaces of the microspheres, separated by a spacing layer, is disposed a reflective layer, e.g., vapor-coated aluminum.
A typical encapsulated-lens retroreflective sheeting comprises a monolayer of microspheres partially embedded in a binder layer with the front surfaces of the microspheres protruding from the binder layer and having a cover film diposed in front of the microspheres, protecting the air to microsphere interface. The rear surfaces of the microspheres have a reflective layer, e.g., vapor-coated aluminum. Examples of encapsulated lens retroreflective sheetings are disclosed, e.g., in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult).
U.S. Pat. No. 3,176,584 (DeVries et al.) discloses that a reinforcing layer may be incorporated into an embedded lens retroreflective sheeting. The reinforcing layer may be of a similar composition as the binder or space coat material in which the microspheres are embedded. The layer may be applied to the back side of the specularly reflective layer via spraying, i.e., by a solvent-coating technique. Examples of the reinforcing layer materials disclosed include methyl methacrylate, flexible epoxy resins, chloro-sulfonated polyethylene, polystyrene, polypropylene, polycarbonate resin, ethyl cellulose, and cellulose acetate-butyrate. DeVries et al. do not discuss the advantages or usefulness of the reinforcing layer for embedded lens sheeting, except to point out that when applied to the contoured reflecting layer, the reinforcing layer provides a flat surface. The specularly reflecting layers of embedded lens retroreflective sheetings such as those disclosed by DeVries et al. are typically very thin, i.e., on the order of 0.06 microns thick, and must be disposed in special relationship to the microspheres in order for the sheeting to provide useful retroreflection. Because the specularly reflecting layers are typically so thin, they are themselves very fragile and do not provide substantial protection to the spacing layer. Thus, the retroreflectivity of the embedded lens sheeting may be impaired by disturbance of the specularly reflective layer and spacing layer as the reinforcing layer is applied. Such disturbances may be particularly critical where the reinforcing layer is applied with high solvent content or at high temperatures that could deform the spacing layer.
Structured film retroreflectors, such as cube-corner retroreflectors, typically comprise a sheet having a generally planar front surface and an array of structured elements protruding from the back surface. Cube-corner reflecting elements comprise generally trihedral structures that have three approximately mutually perpendicular lateral faces meeting in a single corner. In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the structured elements so as to exit the front surface in a direction substantially toward the light source (i.e., it is retroreflected).
The light rays are typically reflected at the structured element faces due to either total internal reflection (TIR), or due to specular reflective coatings such as a vapor-deposited aluminum film. Reflectors relying on total internal reflection require an interface between the faces and a material, typically air, having a lower index of refraction. Examples of cube-corner type reflectors are disclosed in U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No. 4,025,159 (McGrath), U.S. Pat. No. 4,202,600 (Burke et al.), U.S. Pat. No. 4,243,618 (Van Arnam), U.S. Pat. No. 4,349,598 (White), U.S. Pat. No. 4,576,850 (Martens), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appeldorn et al.), and U.S. Pat. No. 4,895,428 (Nelson et al.). Typically, such retroreflective sheetings exhibit a retroreflective brightness (i.e., a coefficient of retroreflection) of greater than about 50 candela/lux/square meter.
In applications in which the retroreflective sheeting is likely to be exposed to moisture (e.g., outdoors or in high humidity), the microsphere lenses can be protected with a cover film and structured retroreflective elements can be encapsulated with a conformable sealing film, as disclosed in U.S. Pat. No. 4,025,159 (McGrath) and U.S. Pat. No. 5,117,304 (Huang). Conventional sealing/cover films may be single or multi-layer thermoplastic films or thermoplastic/thermoset that are attached to the cube-corner surface or located above the microspheres. In the microsphere retroreflective sheeting, the cover film is peferably light transmissive and it prevents contamination of and/or degradation of the microspheres which would inhibit their ability to retroreflect light. In structured retroreflective sheeting, the sealing film may be either opaque or light transmissive and also maintains an air interface at the backside of the structured elements to maintain retroreflectivity due to the refractive index differential between the structured element material and air. The sealing film also protects the surfaces of the structured elements from degradation caused by environmental exposure.
The sealing/cover films are typically attached to the microsphere-coated base sheeting or the structured retroreflective film using a heated embossing tool to create a cellular pattern (i.e., cells) between the sealing/cover films and the microsphere-coated base sheeting or the structured film. That embossing typically leaves the retroreflective sheetings with uneven back surfaces embossed in the cellular pattern used to attach the sealing/cover films.
That uneven back surface can provide the opportunity for humidity-induced construction buckling when the retroreflective sheeting is attached to a substrate such as a signboard because the indentations in the back surface provide channels into which moisture travels. After the moisture is in place between the sheeting and the substrate, expansion and contraction caused by temperature variations can cause localized delamination of the sheeting from the substrate. Although adhesives typically used to attach the sheeting to the substrate can, to some extent, fill in the indentations and reduce moisture penetration, many do not have sufficient compliance or flexibility to do so completely.
Another disadvantage of embossed retroreflective sheetings is that the indentations formed in the sheeting to emboss the cover sheet may weaken the components in the retroreflective sheeting and/or serve as stress concentrators that decrease the peel strength of the sheetings.